In order to improve environmental pollutions due to disposable bowl made of foamable synthetic resin, plastics and aluminum foil, etc., there have been studies on biodegradable disposable bowl consisting of natural polymers such as paper and starch, etc. which can be decomposed after filled in ground.
Since said disposable bowl is biodegradable contrary to such disposable bowl made of synthetic resins etc., it does not cause environmental pollutions and can be easily processed.
However, an inner or outer part of the biodegradable disposable bowl can be polluted with pathogenic Escherichia coli, O-157, pseudomonas aeruginosa, staphylococcus and salmonella, etc. In addition, since the biodegradable disposable bowl can be decomposed by microorganism, it has a very weak preservative property. Also, the biodegradable disposable bowl has a poor impact-resistance compared to that of the prior plastic bowl. Further, the biodegradable disposable bowl has a poor water-resistance and therefore there is such a problem that it is very easy for water to penetrate into the biodegradable disposable bowl.
Accordingly, there were known several techniques for adding the water-resistance, impact-resistance, antibiosis and preservative property, etc. to the biodegradable disposable bowl. The examples are as follow.
Japanese Patent Publication No. Hei 8-311243 discloses a biodegradable foamable composition having improved antibiosis, antifungal property, impact-resistance and water-resistance, which is made by combining starch based polymers, vegetable fibers, metal ions, foaming agent and aliphatic polyester.
Japanese Patent Publication No. Hei 7-97545 discloses that a surface of tray for food made of starch based biodegradable materials deficient in water-resistance is coated with a coating agent having poly L-lactic acid, which is a biodegradable aliphatic polyester, dissolved in CFC 123, which is a halogenated hydrocarbon, thereby improving water-resistance of the surface of the tray.
U.S. Pat. No. 6,361,827 discloses a method for providing water-resistance by a chemical bonding of prolamin such as zein to a surface of polysaccharide molded body.
However, the prior methods for preparing a biodegradable disposable bowl still have such problems that the bowls have poor long-term preservative property and therefore become decomposed due to microorganism etc. particularly when they are used for storing foods. Further, they have problems of being deficient in sterilizing and deodorizing properties and having poor water-resistance, which is particularly required for a disposable bowl for instant noodles.
Also, when an additive is added to a biodegradable composition additionally for improving the water-resistance, it is necessary to regulate a content of the additive. If the content is not properly regulated, it is difficult to easily achieve a desired property such as an overall strength, preservative property, sterilizing property and deodorizing property.
On the other hand, the biodegradable disposal bowl has a low production efficiency due to its poor releasing property.
That is, when preparing the biodegradable disposable bowl, the bowl is not easily released from a mold in case that a depth of the bowl is 5 cm or more. As a result, it is required to manually release the bowl one by one from the mold after discontinuing the preparing process, which lower the production efficiency.